Tension control apparatus for belt storage loop of an extensible belt conveyor



Sept. 27, 1960 J. J. SLOMER 2,9

TENSION CONTROL APPARATUS FOR BELT STORAGE LOOP OF AN EXTENSIBLB BELTCONVEYOR Original Filed Oct. 17, 1957 2 Sheets-Sheet 1 INVENTOR.

SLOMER AT TOR N EY JOSEPH J.

J. J. SL OMER Sept. 27, 1960 V 2,954,114

TENSION CONTROL APPARATUS FOR BELT STORAGE LOOP OF AN EXTENSIBLE BELTCONVEYOR Original Filed Oct. 17, 1957 2 Sheets-Sheet 2 OM o N L W 5 WA HP .E S O J no TENSION CONTROL APPARATUS FOR' BELT I STORAGE. LOOP OF ANEX'I'E'NSIBLE BELT CONVEYOR Joseph JJ Slomer, Chicago, Ell, assignor toGoodman Manufacturiug' Company, Chicago, 111., a corporationof'lllinois' Continuation of abandoned application Ser. No. 690,660,

Oct. 17, 1957. This application Aug. 14, 1958, Ser. No; 755,052

12 Claims.- ((31; 198l--208) The invention relates generally tohydraulic devices for maintaining the proper tension on the belt storageloop of'an extensible belt conveyor, and more particularly to apparatuswhereby a tail section remote from the belt storageloop may bemovedtowards the working face by releasing an hydraulically operated cablewinch for the storage'loop.

Thisapplication is a continuation of my application Serial No; 690,660,filed October 17,' 1957, now abandoned, forTension Control Apparatus forBelt Storage Loop of'an Extensible Conveyor now abandoned.

The invention herein is primarily useful in connection with theapparatus for maintaining the proper tension upon thebelt storage loopdisposed at the head or outby end'of an extensible conveyor. For reasonsof minimizing th'e power requirements it is eminently desirable tooperate thepump supplying the fluid'pressure system at alow'unloadingpressure when the head section has been placed in position. However,after the set up period it may be desirable to move the tail section ashort distance toward the working face. Heretofore, such operationscould be carried on only when the mechanism for maintaining tension onthe belt loop was completely operative in both storageloop takeup andlengthening directions, and if for some reason the mechanism at the headsection was inoperative, the tail section could not be readily moved forany distance without affecting the-operation of thebelt storage loop.For example, if the conveyor was lengthened 'by advancing the tailsection under the conditions described, such advance would impose aninordinate amount of tension on the conveying belt. The invention hereinmakes it possibleto move the tail section toward the working facewithout imposing too great a tension on the belt.

According to the structure disclosed herein it is possible after the setup period of the extensible conveyor tooperate the pump supplyinghydraulic fluid for winding up and maintaining tension upon the beltstorage loop at a low unloading pressure, means being provided forenablingthe belt storage loop to lengthen a short distance withoutoperating the pump when the conveyor is shortened by retreat of the tailsection, and conversely, to enable the belt storage loop to contractupon lengthening of the conveyor in accordance with the advance of theworking face.

Structure according to the present invention makes it possible tomaintain' the conveyor in position with the belt storage loop properlytensioned although the hydraulic equipment at the head section foroperating the belt storage is inoperative. Such hydraulic equipmentincludes locking means for the winch maintaining the proper tension onthe storage loop. If the conveyor is lengthened by advancing the tailsection such locking means is released so that the belt storage loopsmay shorten with the proper tension andsuch tension is maintained'eventhough the pump and'hydr'aulic circuit for the winch is inoperative.

ice

With the foregoing considerations inv mind it is a principal objectofthe invention to provide a novel hydraulic circuit and mechanismcontrolled thereby, whereby the pump. supplyingthe hydraulic circuit'forthe belt storage.

loop canbe unloaded at a low back pressure while at the samevtir'nepermitting offlengthening and'shortening of the belt storage loop asmaybe occasioned by change in length of the conveyor, together withlocking means effective to lock'the storage loop in position, suchlocking means-being releasable when-the hydraulic circuit is in-.

operative and when the conveyor is lengthened.

Another object is to provide locking means'for the belt storage loops,.such locking means being-operable when the pump and hydraulic circuitfor the belt loops is ,in-

operable, as when theconveyoris not operatedbetween.

Working shifts.

Other objects and important features of the invention w1ll be apparentfrom a. study of the specificationfollowpracticing the. principlesthereof."

ings herein, andfsuch other embodiments are intended to bereservedespecially as they fallwithin.th'e scope and? spirit of thesubjoined claims.

In th'edrawings:

7 Fig.1 is an elevational view-of an extensible conveyor. havingtheimproved' hydraulic circuit according, tothepresent inventionembodied therein;

F-ig, 2 isa schematic View ofan hydraulic .circuitZfor maintaining.thebelt: storage. loop of 'Q-the. conveyor seen in Fig; lot the propertension value, said circuit. having the improvements according to thepresent invention -em.--

bodied therein;.and

Fig. 3"is an alternate'embodimentshowingmechanicalmeans for transmittingmotion upon increase intension beyond; :a. predetermined value of thebelt storage loop tocausereleaseof the takeup. winchforsuch beltstorage.

loop. 7

Referring. now to Figl of the drawings, {the improvements according tothe present invention are embodied-Yin.

an extensible conveyor-indicated .by the referencenumeral iltl and.having an :inbfy or. tail lsection..11I and. an.

outby or head'section 12; each being mounted respectively for movementupon crawler treads. 13 'andf14. A conveyor belt h'avinga conveying:reach? 16-and a return.

reachflT-is trained for orbitalmovement betweenthe endsectitins 11 .and12, driving. means forthe belt and. means. for reversing thereaches ofthe belt at thetail. section 11 and the .headsection 12'not. being.shown. since a Winch 21. at the. tail section. 11,. each strand beinggui ded by idler sheaves 22.and .23 at thetail section 11. The load on.the conveying. reach". 16 is transferred. into thestrands 18 bytroughing roller. assemblies 24, and. the load on the strands 18 istransferredby means-ofi standards'26into the minefioor, eachstandardhaving. areturn idler 27 for guiding and supporting. thereturnreach'.17l

As seen in Fig. 1, the standard26 inby of the head section 19 -providesasupport for a pair of spaced'rails' 28, the outby ends of which aresupported'at the head section 12. The spacedrails 28 atford'asupportfor: a fixed idler'carriage 29 and a movable idler .carriag e.31,[carriage 29 having idler sheaves 32and 33', and movable. idler carriage.31 having idler pulleys 34'and.'36,'.the-re-- turn reach" of the belt17 being reeved' between the afore'- Patented Sept. 27, 1960 said idlerpulleys to define a plurality of belt storage loops 35. The movablecarriage 31 is part of a combined belt slack takeup and 'tensioningmeans 30 and includes an idler sheave 37 for training draft means, suchas a belt tensioning cable 38, having one end wound upon a storage winch39 forming part of power means 40 to move sheave 37 and pulleys 34 and36 in a direction to en large belt storage loops 35. The cable 38 istrained around the idler sheave 37, and also around fixed idler sheaves41 and 42 on the head section 12, see also Fig. 2, the other end of thecable 38 terminating in a hook 43 which is attached to a rod 44 having amovable flange 46 at one end thereof to provide a movable abutment for acoil spring 47. A cylindrical guide 48 for the rod 44, the flange 46 andspring 47 has a closed end 49 against which the other end of spring 47is bottomed. Rod 44 and spring 47 form part of sensing means 45 of aforce urging the pulleys 34 and 36, and carriage '31 in a direction tocontract belt storage loops 35.

As see min Fig. 2, the power means 40 are provided for winding the winch39 in a direction so as to impose a proper degree of tension upon theconveying reach 16 and the return reach 17 by imposing a pull on thebelt storage loops 35 'reeved between the idler pulleys of the fixedidler carriage 29 and the movable idler carriage 31. Such power means 40includes a pump 51 connected by a line 52 to a supply of hydraulicfluid. The output from the pump 51 is connected by a line 53 to anunloading valve 54 having a working pressure line 56 leading therefrom.A line 57 branches from pressure line 56 and is connected to anaccumulator 58. A check valve 59 is. connected in the working line 56 toprevent back flow from the accumulator 58 through valve 54 to tank.

The unloading valve 54 is arranged to bypass fluid to a tank line 61when the pressure within the accumulator 58 reaches a predeterminedupper pressure limit, the unloading valve 54 being of the type to imposesubstantially no back pressure against thepurnp 51 when it is moved tothe unloading position.

A control valve indicated generally by the reference numeral 60 isarranged to control the flow of fluid to a fluid motor 62 to drive theWinch 39 in directions to impose tension on the belt storage loop. Thefluid motor 62 drives a pinion 63 meshing with a gear 64 fast with aworm 66. A worm wheel 67 meshes with the worm 66, and is fast with apinion 68 in turn meshing with a gear 69 fast on a shaft 71 turning thewinch drum 39.

Locking means 70 preventing unintended unwinding of the drum 39 includesa ratchet wheel 72 fast to the worm 66, and cooperating with a pawl 73pivoted at 74. The pawl 73 is urged into contact with the ratchet wheel72 by means of a spring 76 bottomed around a boss 75 on the pawl 73, theother end of the spring being bottomed at the bottom of a recess 77 inan abutment 78. p

During operation of the pump 51, whether it operates against low backpressure during unloading of the unloading valve 54, or whether itoperates against a high back pressure when valve 54 is loading, the pawl73 is urged out of engagementwith the ratchet wheel 72 by means of apiston 79 engaging an arm 81 of the pawl 73. Piston 79 is slidablewithin a bore 82 formed in the abutment 78 and the bore 82 is connectedby means of a line 83 to a valve body 84 communicating with the outputof pump 51 by means of a pressure line 86 branching from the output line53. Pressure line 86 is connected to a chamber 87 of a servo cylinder 88having a piston 89 provided with an extension 91.

Under conditions as will appear, auxiliary means 90 is provided torelease the pawl 73- when spring 47 is shortened by a contraction ofbelt storage loop 35, and such auxiliary means 90 includes an arm 92carried by the rod '44 to make contact with the extension 91 and d urgethe piston 89 against the force of a light return spring 93.

A line 94 connects the chamber 87 to the valve body 84, and normally thepressure from the pump 51 is manifested past a check valve 96, so thatthe pressure is effective against the piston 79 to rock pawl 73 out ofengagement with ratchet wheel 72. When the pump 51 no longer operates,the load in the spring 77 is normally eifective to urge the piston 79 tothe position shown in Fig. 2, the flow incident to the movement of thepiston 79 moving past a throttling valve 97.

The control valve 60 includes a valve body 98 having a bore 99 thereinwith a spool valve plunger 101 slidable in valve bore 99. The spoolvalve member 101 is provided with spaced lands 102, 103 and 104, theselands being spaced by portions of reduced diameter 106 and 107. In theneutral position of the valve spool shown the land 103 blocks pressureline 56 connected to the valve body 91 at a port 108.

During set-up of the conveyor seen in Fig. 1, the valve spool 181 ismoved to a winding position, where the land 103 unlaps the port 108 tocause communication of high pressure line 56 with a line 109 connectedto the fluid motor 62. The exhaust fluid from the motor 62 moves by wayof a line 111 to the valve 60 and past the reduced diameter portion 107and to a passageway 112 within valve body 98 connected to a conduit 113.

A reverse flow valve indicated generally by the reference numeral 115includes a valve body 116 having a bore 117 therein receiving a spoolvalve member 118 havmg a pair of opposed end lands 119 and 121 separatedby a portion of reduced diameter 122. When motor 62 is operated asdescribed to wind the cable 38 upon the winch drum 39 and thereby imposetension on the storage loop 35, the exhaust fluid from the motor 62 iseifective in a passageway 123 in the valve body 116 and connected toconduit 113 to shift the spool valve member 118 to the right as shown.

Exhaust fluid from the motor 62 is ported to tank through reverse flowvalve 115 past a check valve 124 in the valve body 116, the check valve124 being disposed across a passageway 126 connected to a passageway 127in turn connected to tank as shown.

The reverse flow valve 115 includes a high pressure relief valve 128which is connected by means of a line 129 branching from the pressureline 56, line 129 being connected to an annular port 13 1 to which therelief valve 128 is connected, the discharge side of the valve 128 beingconnected by a passageway 132 in the valve body 116 to tank as shown.

Normally the setting of the high pressure relief valve 128 within theshuttle valve 115 may be of the order of 1,000 p.s.i., while theunloading valve 54 is set to load at a value of 775 p.s.i., and tounload at a value of 900 p.s.i. It will be seen, therefore, that thehigh pressure relief valve 128 is operable only when the unloading valve54 fails to operate for some reason, and only when the control valve 60is shifted to the winding position.

It may be noted that during the winding operation just described theoutput from the pump 51 is effective past the check valve 96 to move thepiston 79 and the pawl 73 from the ratchet wheel '72 so that the motor62 is freely released to drive winch drum 39.

The system thus far described is effective also to cause the reel 39 toturn in an unwinding direction, so that the tension on the storage loops35 can be released, as may be indicated for the removal of a beltsection from the conveyor 10.

Under such condition of operation the valve spool 101 is moved to theunwind position. This will cause the land 103 to lap the port 109, andcause pressure port 108 to communicate past the reduced diameter portion107 with the line 111 to drive the motor 62 in the opposite or unwindingdirection. At such time line 109 will become an exhaust line and theexhaust fluid will move .pastasset 14 reduced diameter portion 106 tothe exhaust passageway 113'causring the valve spool 118 to adopt theposition as during the winding operation. It may be noted that theland104'at such time blocks the passageway 112 so that the pressurefluid is diverted only into the line 111.

As before, with the winding operation, pawl 73 will release ratchet 72,the pressure from pump 51 being efiective past servo valve cylinder 8%and valve 84 to move piston 79"an'd pawl 73. Since the pawl 73 isreleased, motor 62 can be driven in the reverse direction to release thetension on'the belt storage loops. As was described in the objects ofthis invention, it may be necessary to move the tail section 11 towardthe headsection 12 a limited distance or away from the head section12requiring the respective lengthening or shortening of' thestorage loops35. Under such described operation, and when the pump 51is eitherloading or un loading, the control valve 60 will be left in the windposition. As was previously stated, the pawl 73 is released bythepressure from pump 51.

Consider now the functions obtaining when the belt storage loop 35 mustbe shortened to enable the tail section'11 to advance without imposingtoo great a tension on the reaches 16 and 17 of the conveyor belt. Whenthe storage loop 35 is shortened, motor 62 is driven as a pumpdn thereverse direction. At this time line 109 becomes the 'out'put line from?the motor 62 while acting as apump and line 111 becomes a supply linethereto. The output'from motor 62 while acting as a pump moves pastreduced diameter portion 106 and land 103, which now unlaps port 168.Land 102 laps passageway 113 atbore 9?. Line 111 is now connected pastreduced diameter portion107 to line 112 and 113. Since the line 111 atsuch time will be at a suction pressure, it will shift th'e' sp'oolvalve member 118 to the left thereby unlapping a port133 therein. Fluidwill be supplied to the motor 62 while. acting as apump from the tankline 127 'past acheck valve 134 connected to port 133, and by a line 136connected to the passageway 123-. At the same time the fluid pumped'bymotor 6-2 (now acting as a pump) will move from line 109 past the land103 (which now unlaps the port 108), the pumped fluid moving by way ofline 129 andpast the port 13-1 and past the reduced diameter portion122,the pumped fluid moving by a passageway 135past a low pressure reliefvalve 137, it being connected across-the passageway 132.

Accumulator 58 at this time is also being released of its pressure pastlow pressure relief valve 137.

By way of example, low pressure relief valve 13-7 may beiset'at thevalue of approximately 200 psi It will be seen, therefore, that themotor 62, acting as a pump operates against a lower back pressurethereby preventingathe'b'elt tension from reaching too high a value.

Consider now the condition obtaining when the tail section 11 is movedmore than a limited distance toward the head section 12 which wouldrequire a lengthening of the belt storage loops 35 to prevent sag ofboth the conveying reach'16 and the return reach 17, all during whichmovement it is'desirable to maintain the same: tension upon the belt.The apparatus according to the present invention is effective to providesuch lengthening'of the belt storage loop 35 and to maintainthe'predetermined tension for a limited degree of movement of tailsection 11 while the pump 51 is in either the unloading or loadingoperation.

The control valve Gilremains in the'wind position as before, and theaccumulator 58 and pump 51 are effective to supply pressure fluid pastthe land 103 and into the line109"to"drive' the-motor 62 in a windingdirection, exhaust fluid from the motor being exhausted past the reduceddiameter portion 107 of valve spool 101. Spent fluid is exhausted by wayof the passageways 112 and line 113, valve spool 118 of shuttle valve115 being shifted to the right, as during the original winding operationdescribed. The exhaust fluid then moves past'the checkvalve124 andthrough line 126 as previously described totank by line 127.

When the conveyor is no longer operating, as between working shifts,proper tension is maintained uponthe belt storage loop. The stopping ofpump Sleventually results in the pressure in the system dropping toanineffective working value. When pump 51 is inoperative, no pressure ismanifested in lines 86 and :94, and thepawl 73' is moved to thelocking-position by'the spring-76;

The belt storage loop.35 is thus heldat its proper tension value, and aslong as head and tail sections 12 and 11 are maintained in theirrelativepositions, suchtension will'be maintained. If the tail section 11 ismoved away from head section 12 excessive tension on the belt isprevented. The resultant shortening of the belt storage loops 35 causesthe idler-sheave to move to the right thereby shortening spring 47 ofaux iliary means 90 causing arm 92 to move piston89 past thepoint whereline86 is connectedto chamber 87. Continued movement of piston '89 byarm 92' causes a pulse'of fluid to move past check valve-94andraisepiston 79 torelease pawl 73.

Winch '39 is then free to pay off 'the-cable38, motor 62 operating as apump against the baokpressure of valve 137ofreverse flow valve 115. Whenspring 47 lengthens 1 once more and pistons 89 returns under urgence ofspring 93, the pawl is once more locked.

Referring now'to Fig. 3 of the drawings, there is shown anotherarrangement whereby the pawl 73' can be -moved to a position to releasethe ratchet 72. In this -arrangement the pressure in the line 86 when.the pump 51 is operating is directly effective against the piston- 79.When pump 51 is inoperative, instead of the arm92 providing a pulse ofpressure fluid when the belt storage loops are over tensioned, it iseffective to push a 'Bowden cable 141. Bowden cable 141 has an endmember 142 slidable within a sheath for the cable 141, the other endofthe cable having a solid member 144 =eifective against an extension 146'of the piston 79.

From the description foregoing it is believed evident thafthere has beenprovided a new and improved arrangement for controlling the tension ofthe conveyor belt of an extension conveyor.

The 'arrangement described makes unnecessary the continuous'operation ofthe pump at a high loading pressure which? would unduly heat thehydraulic fluid, andtheapparatus is so constructed that the ordinarilyexperienced limited amounts of take-up and slackening off -ofthe beltmaybe provided at all times while operating-the pump at the headsectiononly' at the lower unloading pressure. The locking means isadapted to be released during *idleperiods of the pump when the storageloop is over-tensioned, so that the excess of tension can be released;

While the invention has been described in terms of some preferredembodiments thereof its scope is intended to be limited only by theclaims here appended.

I claim as my invention:

1. In an extensible conveyor having a belt storage loop which may belengthened and shortened in accordance with the length of said conveyor,said belt storage loop including at least one movable pulley havinga'portion of said belt storage loop reeved thereabout, the improvementin'said conveyor for maintaining-saidbelt storage loop at desiredtension which oomprises a' pumpconnected to a source of motive fluid,belt slack take-up .and tensioning means including afluid-'motor-operated-winch having draft means connected to said pulleyfor moving said pulley in a direction to lengthen and apply tension tosaid loop, means connecting said pump and said fiuid-motor-oper-ated-Winch, locking means effective to lock said fluid-motoroperated-winchagainst movement in a direction to release the tension in said loop,fluid operated means for releasing said locking means, sensing meanseffective to sense a force on said pulley urging said pulley in adirection to contract said loop, said sensing means including a pistonand cylinder, and means rendering the piston and cylinder effective tooperate said fluid operated means when the tension in said storage loopincreases beyond a predetermined amount.

2. The invention as defined in claim 1 wherein the fluid motor for saidwinch operates as a pump during release of tension on said belt storageloop, and wherein means are provided for reducing the back pressure onsaid fluid motor when so operating.

3. The invention as defined in claim 1 further characterized .firstly,in that the locking means includes a ratchet and pawl and, secondly,that the fluid operated means includes a second piston and a secondcylinder connected to the output of the pump, said second piston beingefifective to rock the pawl to a release position in response to a forceof predetermined magnitude, as sensed by the sensing means, urging saidpulley in a direction to contract the loop.

4. The invention as described in claim 1 further including means foractuating the fluid operated means in response to operation of the pumpto thereby release the locking means.

5. In an extensible conveyor according to claim 1, means biasing saidfluid operated means in a direction to lock said locking means, and afluid connection between said pump and said fluid operated meansenabling said pump when operating to release said locking means.

6. In an extensible conveyor having a belt storage loop which may belengthened and shortened in accordance with the length of said conveyor,said belt storage loop including at least one movable pulley having aportion of said belt storage loop reeved thereabout, the improvement insaid conveyor for maintaining said belt storage loop at desired tensionwhich comprises a pump connected to a source of motive fluid, belt slacktake-up and tensioning means including a fluid-motor-operated-winchhaving draft means connected to said pulley for moving said pulley in adirection to lengthen and apply tension to said loop, means connectingsaid pump and said fluidmotor-operated-winch, locking means effective tolock said fluid-motor-operated-winch against movement in a direction torelease the tension in said loop, releasing means for releasing saidlocking means, sensing means having an element movable in accordancewithvariation of tension in said loop, said element being operativelyconnected to said releasing means to move the latter to release saidlocking means at a position of said element corresponding to apredetermined maximum tension in said loop.

7 7. In an extensible conveyor according to claim 6.

wherein said releasing means includes fluid operated means for releasingsaid locking means and a fluid 'connection between said pump and fluidoperated means effective to release said locking means in response tooperation of said pump.

8. -In an extensible conveyor having a belt storage loop which maybelengthened and shortened in accordance with the length of said conveyor,said belt storage loop including at least one movable member having aportion of said belt storage loop reeved thereabout, the improvement insaid conveyor for maintaining said belt storage loop at desired tensionwhich includes belt slack takeup and tensioning means, locking meansefiective to prevent release of the tension in said storage loop,releasing means for releasing said locking means, and sensing meanseffective to sense movement of said movable memher in a direction tocontract said storage loop, said sensing means including means foreffectively operating said releasing means to release the locking meanswhen the tension in the storage loop reaches a predetermined amount.

. 9. In an extensible conveyor having a belt storage loop which may belengthened and shortened in accordance with the length of said conveyor,said belt storage loop including at least one movable member having apor-' tion of said belt loop reeved thereabout, the improve ment in saidconveyor for maintaining said loop at desired tension which includesbelt slack take-up and tensioning means effective to move the movablemember in a direction to lengthen said loop and to apply tensionthereto, locking means effective to lock said take-up and tensioningmeans so as to prevent release of the tension in said loop, releasingmeans for releasing said locking means, and sensing means having anelement movable in response to variation of tension in said loop, saidelement being operably connected to said releasing means and effectiveto cause the releasing means to release said locking means at a positionof said element that corresponds to a predetermined maximum tension insaid loop.

10. A belt tension control system for use in extensible belt conveyor ofthe type in which a belt is reeved about at least one movable pulley toform a belt storage loop which may be lengthened and shortened inaccordance with the length of the conveyor, said system including, incombination, a winch and a cable reeved about the winch and operativelyconnected to the pulley to move the pulley back and forth to maintain adesired tension in the belt storage loop and consequently in theconveyor belt, power means for rotating the winch to reeve in cable, aratchet and pawl locking mechanism operatively engaging the winch andeffective to prevent unwinding movement of the winch, a spring biased tourge the pawl into locking engagement with the ratchet, a cylinderconnected to a pump, a piston in the cylinder effective to urge the pawlout of engagement with the ratchet against the spring in response to agiven pump pressure, a belt tension sensing member movable in responseto an increase in tension in the belt storage loop operable toeffectively urge the pawl out of engagement with the ratchet when apredetermined tension is reached in the belt storage loop, and arestoring spring biased to urge the belt tension sensing member out ofeffective engagement with the pawl.

, 11. The belt tension control system of claim 10 further including aforce transmitting cable assembly disposed between the belt tensionsensing member and the pawl, said cable assembly being effective to urgethe pawl engaging piston against the pawl to thereby release the lockingmeans when a predetermined tension, as sensed by the belt tensionsensing member, is reached in the belt storage loop.

12. The belt tension control system of claim 10 further including anauxiliary cylinder in the line between the pump and first cylinder, saidauxiliary cylinder having a piston with a portion engageable with thebelt tension sensing member, said auxiliary piston being effective, whenengaged by the belt tension-sensing member, to urge a shot of pressurefluid against the first piston to thereby disengage the pawl from theratchet.

References Cited in the file of this patent UNITED STATES PATENTS2,788,116 Wood Apr. 9, 1957

